docs: correct solo-mission count 21 -> 20 (level_54 'Cuba' is the ending-credits sequence, not a mission)

The campaign has 20 playable missions (SP_LEVEL_MAX // 20 in
bondconstants.h; mission_folder_setup_entries in front.c has exactly 20
entries). The old '21' counted -level_54 (LEVELID_CUBA), which the front
end jumps to after the final mission for the extended cast display +
credits scroll (do_extended_cast_display / bondviewRenderCredits, D129) -
it is not a selectable mission.

- README, docs/index.md, .github/release-notes.md,
  docs/security-and-fidelity-status.md: 21 -> 20 in current-status claims
- AGENTS.md phase status: same correction
- docs/dev/LEVEL-STATUS.md + LEVEL-PLAYTEST.md: relabel the level list;
  counting note explains historical '21/21 PASS' sweep results = 21 stage
  boots (20 missions + ending sequence)
- social-preview card: stat line 21 -> 20, PNG regenerated (1280x640)

.github/release-notes.md

@@ -8,7 +8,7 @@
 > The full single-player campaign runs at a steady 60 fps with audio (music +
 > SFX) playing throughout, on Windows and Linux including Steam Deck, and it
 > is completable end to end: the whole campaign has been playtested through
-> all 21 missions (Agent difficulty). It's an early public cut: all 21 solo
+> all 20 missions (Agent difficulty). It's an early public cut: all 20 solo
 > missions load and run crash-free, but the known issues below are real;
 > feedback is very welcome. **This is a pre-1.0 release, not a finished
 > product** — expect rough edges and missing features until v1.0; see the